Complement Depletion and Pyelonephritis. Unilateral acute pyelonephritis was produced by intravenous administration of E. coli (strain IMRU-54) in complement depleted and left ureter ligated rats. The complement was depleted by repeated intraperitoneal injection of 1 ml of solution containing 18 units of the inhibitor every 5 hours. Inhibition of immigration of leukocytes into the kidney was observed in complement depleted rats on days 1, 1 and 1/2, and 2 following the injection of E. coli. This inhibition of leukocyte immigration appeared to be associated with better structural preservation of the experimental kidneys than that of the controls. The data appear to indicate that the leukocyte infiltration into the E. coli infected kidney may play a role in the destruction of renal parenchymal tissue. This protective action of complement depletion was not demonstrable beyond a day 2 experimental time period. (The inhibitor refers to cobra venom factor). Site of Bacterial Colonization in Acute Pyelonephritis. The initial site of bacterial colonization in the kidney of rats that were intraperitoneally or locally injected with E. coli (2 times 10 to the 8th power), was intratubular lumina. The bacteria appeared to have preferentially adhered to the brush border, and to have entered into the cytoplasm of proximal convoluted tubular epithelia, and further to have invaded into the interstitium from the tubular lumina. The tubular lumina may provide an initial and temporary shelter for the bacteria to replicate.